This invention relates to resin compositions. More particularly, it relates to filled resin compositions containing atactic polypropylene.
Filled resin compositions are well known in the resin industry. "Filling" resin with an organic or inorganic filler is an accepted method of "stretching" the available supply of a particular resin or of producing a resin composition at a lower cost per unit. Whittington's "Dictionary of Plastics" (Technomic Publishing Co., Stamford, CT., 1968) discusses fillers.
The preparation of a mixture of isotactic and atactic polypropylenes is disclosed in U.S. Pat. No. 3,640,921. Other U.S. patents, such as U.S. Pat. Nos. 2,927,047; 2,956,042; 3,220,966; 3,261,889; 3,370,106; and 3,375,303 are typical of the prior art concerning the use of atactic polypropylene with other resins.
Filling a resin, such as low density polyethylene, with a filler, such as limestone or calcium carbonate, results in a composition that typically has a melt index lower than the melt index of the virgin resin. Thus, even though filled resins may have certain economic advantages, one of the main disadvantages arises during the extrusion of these resins. The melt index of the filled resin is lower than that of the virgin resin, the amount of lowering depending on the percentage of filler in the finished resin composition. Materials with a lowered melt index require special attention during extrusion. One method of raising the melt index of such a finished filled resin is to raise the extruder temperature, but this can result in thermal decomposition or some associated degradation, of the resin, thus giving an inferior product.